The planes are used to supplement the FBI’s ground-based surveillance techniques and are believed to have been in operation for some years. More recently, however, technological advances have meant the spy planes have been equipped with higher resolution cameras and mobile phone tracking capabilities.

In total, the AP traced more than 100 flights over a 30-day period to FBI surveillance, operating in both urban and rural areas. Many of the planes were registered to firms that do not actually exist such as KQM Aviation, FVX Research, NBR Aviation and PXW Services.

Although it is important law enforcement officials are given the tools to apprehend criminals, the report will increase fears that the privacy of ordinary citizens is being compromised.

However, despite the FBI proclaiming that its use of surveillance planes is no secret, the agency did request that the AP report was not published as it would mean tax payers would have to cover the cost of creating new cover companies.

"The FBI's aviation program is not secret," FBI spokesman Christopher Allen explained. "Specific aircraft and their capabilities are protected for operational security purposes."

The report also found that most of the aircraft were registered to someone going by the name “Robert Lindley,” listed as chief executive. However, the FBI would not reveal whether Lindley is a government employee and the Associated Press were unable to reach him at any of his listed telephone numbers.

Its use of aircraft is not the only thing the FBI has been criticised for recently. The US law enforcement agency also deployed licence plate readers on a mass scale despite reservations about the impact this would have on public privacy.